The outage in Amazon's Elastic Compute Cloud service last week highlighted the limitations of load balancing and failover systems designed to keep applications running in case of failure. But Amazon isn't the only cloud vendor whose systems can't guarantee 100% uptime.

Building cloud-based applications that can fail over from one data center to another is difficult and may require the customer to have sophisticated technology expertise. Customers may have to work closely with the cloud vendor and purchase third-party load-balancing products to keep applications running in the event of failures like the one that hit Amazon.

GoGrid, which offers infrastructure-as-a-service computing in a fashion similar to Amazon's, offers service credits to customers when uptime falls below 100%, but that doesn't mean the cloud service never goes down.

"For the service elements we deliver, we're saying that we expect them to be up 100% of the time, and if they're not were going to compensate you," says GoGrid CEO and founder John Keagy. "Things do fail. Customers should not interpret a 100% service-level commitment as a 100% service-level guarantee."

But customers can keep their applications running through downtime if they are willing to put some extra work into it, Keagy says. Amazon customers who didn't have robust disaster recovery and failover plans were more likely to suffer downtime last week than those who planned ahead, he says.

GoGrid's cloud offerings are spread across 11 data centers, mostly run by co-location providers. Customers that want applications to fail over from one data center to another can use global traffic management products made by third parties, Keagy says. Customers can also achieve this extra level of protection entirely through services offered by GoGrid, but this "has to be architected in conjunction with us to get that done," Keagy says.

"That's what infrastructure is all about," Keagy says. "This is not platform as a service or software as a service. This is raw infrastructure that requires the user to have some responsibility for how they implement things."

While sites including Foursquare, Reddit, Quora and Hootsuite went offline, the success of photo-sharing site SmugMug shows how planning ahead can help customers survive what SmugMug CEO Don MacAskill called the "Amazonpocalypse."

SmugMug spread across three availability zones, and decided not to use Amazon's "Elastic Block Storage" service because of "unpredictable performance and sketchy durability," MacAskill wrote in his blog. The storage service played a key role in last week's failure.

If you're putting mission-critical applications in the cloud, MacAskill advises spreading them across multiple Amazon regions (East Coast and West Coast, for example) or multiple cloud providers.

Amazon's load-balancing service doesn't work across regions, so customers have to do some extra work on their own and use third-party software to make it happen, says Gartner analyst Drue Reeves. Spreading applications across multiple cloud vendors, meanwhile, is not impossible but difficult due to a lack of standards and interoperability.

Rackspace, another infrastructure-as-a-service provider, recently began offering a Cloud Load Balancers service that protects applications against the failure of a single server. But the load balancer does not spread applications across different data centers.

Josh Odom, who leads product development for Rackspace's cloud platform, notes that running an application in multiple data centers is the best way to guarantee 100% uptime, and Rackspace tries to make it easy for customers to use third-party load balancing and failover products to achieve that.

Rackspace's Texas data center suffered a few power outages in 2009, forcing the company to issue service credits to customers. The company has since brought in new data center experts and performed top-to-bottom audits of the facilities, Odom says. Despite past problems, Odom says Rackspace data centers are designed to withstand "catastrophic failures" including the loss of major power sources or network capacity.

While disaster recovery planning in infrastructure as a service requires some tech expertise, not all cloud services are geared toward the experts. Platform-as-a-service offerings -- such as Microsoft's Windows Azure or Google App Engine -- are designed to minimize involvement with underlying infrastructure and provide developers a relatively simple way to build and host Web applications.

But load balancing and the ability to fail over from one data center to another is still a big plus in platform-as-a-service clouds.

Microsoft recently announced “Windows Azure Traffic Manager,” saying it will allow “deployment of the same application to topologically dispersed data centers enabling the distribution of workload between these data centers through round robin, failover and performance based load balancing schemes.” Azure Traffic Manager is available only in a community technology preview, meaning it’s not ready for all customers. While Windows Azure Traffic Manager distributes traffic across multiple data centers, SQL Azure Data Sync, also in beta, replicates "databases across multiple data centers to prevent against a DC getting lost," according to Microsoft.

Developer Robert McLaws reports on Twitter that, even without Windows Azure Traffic Manager, customers can build applications to fail over across data centers if you "manage it yourself in code."

Google’s App Engine service can shift both applications and data from one data center to another without data loss or downtime in the event of failure, said Google product manager Greg D'alesandre. Google would not say how far apart the data centers are, but said "the system is designed so that there is no single geographic point of failure."

Amazon, meanwhile, has been accused of not providing a full explanation of what actually went wrong last week. Amazon blamed a "networking event" that "triggered a large amount of re-mirroring" of storage volume, creating a capacity shortage, and lost connections to virtual machines.

Thorsten von Eicken, CTO and founder of RightScale, which provides services that enhance the functionality of Amazon EC2, said Amazon "earns an F" for communication and has failed to offer a root-cause analysis.